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Abstract

This study investigated the location and diel habitat preferences (at 100 m reach scale) of fish in a small tributary stream in late spring, early summer. During the day, coho (Oncorhynchus kisutch) preferred areas with more cover (deeper, greater extent of undercut banks) vs. night when LWD was preferred (Pearson correlation and step-wise MLR). Chinook (O. tshawytscha) exhibited an opposite pattern, preferring LWD during the day vs. higher velocity at night. This suggests these two potadromous species may be partitioning resources. Pooling coho, chinook and rainbow trout (O. mykiss) indicated reaches with more LWD were selected at night (r2=0.86, p=0.005) vs. deeper reaches during the day (r2=0.62, p=0.04). Although not measured specifically, we believe LWD supports more macroinvertebrate production vs. the predominantly sandy substrate. Thus, a potential mechanism behind the observed patterns in reach selection may be the tradeoff between food resource abundance vs. predation risk. The majority of captured fish were juveniles supporting the premise first order tributaries can serve as important nursery habitats, especially if they exhibit stable flow and thermal regimes.